As a cathode active material contained in a positive electrode of a lithium ion secondary battery, a lithium-containing composite oxide, particularly LiCoO2, is well known. However, in recent years, for a lithium ion secondary battery for portable electronic instruments or for vehicles, downsizing and weight saving are required, and a further improvement in the discharge capacity of a lithium ion secondary battery per unit mass of the cathode active material (hereinafter sometimes referred to simply as the discharge capacity) is required.
As a cathode active material which may be able to further increase the discharge capacity of a lithium ion secondary battery, a cathode active material having high Li and Mn contents i.e. a so-called lithium rich cathode active material has attracted attention. However, a lithium ion secondary battery using such a lithium rich cathode active material has a problem such that the characteristics to maintain the charge and discharge capacity at the time of repeating a charge and discharge cycle (hereinafter referred to as the cycle characteristics) tend to decrease.
As a lithium rich cathode active material, the following (1) has been proposed.
(1) A cathode active material, which is obtained by mixing a transition metal precursor containing a composite transition metal-compound represented by MnaMb(OH1-x)2, wherein M is at least two selected from the group consisting of Ni, Co, Mn, Al, Cu, Fe, Mg, B, Cr and second series transition metals, 0.4≤a≤1, 0≤b≤0.6, a+b≤1, and 0<x<0.5, and a saccharide, with a lithium precursor, and firing the mixture in an oxidizing atmosphere (Patent Document 1).
Patent Document 1 discloses that by using the above-obtained lithium rich cathode active material, it is possible to obtain a lithium ion secondary battery excellent in the rate characteristics, life properties and charge and discharge efficiency.